Looking for Z' bosons in Supersymmetric E_6 Models through Electroweak Precision Data

Abstract

We review constraints on additional Z' bosons predicted in supersymmetric (SUSY) E_6 models from electroweak experiments - Z-pole experiments, mW measurements and the low-energy neutral current (LENC) experiments. Four representative models - χ,ψ, η, νmodels - are studied in some detail. We find that the improved data of parity violation in cesium atom, which is 2.2-σaway from the Standard Model (SM) prediction, could be explained by the exchange of the heavy mass eigenstate Z_2 in the intermediate state. The improvement over the SM can be found in χ, η, νmodels, where the total χ^2 of the fit to the 26 data points decreases by about five units, owing to the better fit to the atomic parity violation. Impacts of the kinetic mixing between the U(1)_Y and U(1)' gauge bosons on the χ^2-analysis are studied. We find that the Z' model with (β_E, δ)=(-π/4,0.2), where β_E is the mixing angle between Z_χand Z_ψbosons and δdenotes the kinetic mixing, shows the most excellent fit to the data: the total χ^2 decreases by about seven units as compared to the SM. We introduce the effective mixing parameter ζ, a combination of the mass and the kinetic mixing parameters. The 95% CL lower mass bound of Z_2 can be shown as a function of ζ. A theoretical prediction on ζand the U(1)'gauge coupling g_E is studied for the χ,ψ,ηand νmodels by assuming the minimal particle content of the SUSY E_6 models.